The present invention relates to gas turbine engines, and more particularly, to a differential gear system for use in gas turbine engines.
Gas turbine engines include compressors in flow communication with one another. The compressors are disposed upstream of a combustor and high and low speed turbines. The components are disposed about a centerline axis of the gas turbine engine in flow communication with one another. The low pressure compressor is driven by the low speed turbine while the high pressure compressor is driven by the high speed turbine. This process is achieved by air, which is compressed by the compressors, and is then is mixed with fuel and ignited in the combustor to generate hot combustion gases that flow along and turn the high and low speed turbines. The low and high speed turbines not only drive the low and high pressure compressors and produce thrust, but also can be harnessed to produce power that can be used for auxiliary purposes.
Because the rotating components of gas turbine engines require different speeds and power parameters in order to achieve greater efficiencies, gas turbine engines typically operate with high pressure spools comprising the combustor, the high pressure compressor and the high speed turbine (coupled together by a high speed shaft). The high pressure spool is ideal for driving electrical generators, in addition to the high pressure compressor.
Similarly, gas turbine engines use one or more arrays of fans to accelerate the bypass air through a fan section and thereby produce a significant portion of the thrust output for the engine. However, the rotational speed of the fans within the gas turbine engine is limited, and is much slower than that of the core compressor. Thus, gas turbine engines may have a low pressure spool that is coupled to the fan via a gearbox, the low pressure spool comprising the low pressure compressor and low speed turbine (coupled together by a low speed shaft).
For efficiency reasons it is desirable for gas turbine engines to utilize an intermediate pressure turbine to drive a generator independently. Such an arrangement can be achieved by the use of gear assemblies such as epicyclic gear systems. Epicyclic gear systems are complex mechanisms for reducing or increasing the rotational speed between two rotating shafts or rotors. The compactness of planetary or star system type epicyclic gear systems makes them appealing for use in aircraft engines.
Although not unknown in gas turbine engines, the use of differential epicyclic gear systems, where none of the differential elements such a sun gear, ring gear, planet gears, and a planet carrier are tied to a stator ground, is not prevalent. The lack of general application of differential gear systems in gas turbine engines can in part be attributed to conventional differential gear systems inability to provide for a stable and predictable direction of rotation for shafts and rotors coupled thereto. Therefore, during operation of the gas turbine engine an undesirable direction of rotation for the rotors and shafts (and components coupled thereto) can result if a conventional differential gear system is utilized.